The Effects of Different Miniscrew Thread Designs and Force Directions on Stress Distribution by 3-dimensional Finite Element Analysis.

STATEMENT OF THE PROBLEM
The use of miniscrew as an absolute anchorage device in clinical orthodontics is growing increasingly. Many attempts have been made to reduce the size, to improve the design, and to increase the stability of miniscrew.


PURPOSE
The purpose of this study was to determine the effects of different thread shapes and force directions of orthodontic miniscrew on stress distribution in the supporting bone structure.


MATERIALS AND METHOD
A three-dimensional finite element analysis was used. A 200-cN force in three angles (0°, 45°, and 90°) was applied on the head of the miniscrew. The stress distribution between twelve thread shapes was investigated as categorized in four main groups; buttress, reverse buttress, square, and V-shape.


RESULTS
Stress distribution was not significantly different among different thread shapes. The maximum amount of bone stress at force angles 0°, 45°, and 90° were 38.90, 30.57 and 6.62 MPa, respectively. Analyzing the von Mises stress values showed that in all models, the maximum stress was concentrated on the lowest diameter of the shank, especially the part that was in the soft tissue and cervical cortical bone regions.


CONCLUSION
There was no relation between thread shapes and von Mises stress distribution in the bone; however, different force angles could affect the von Mises stress in the bone and miniscrew.


Introduction
Orthodontic miniscrews have revolutionized orthodontic treatment plans. Nowadays, the use of miniscrew as an absolute anchorage in clinical orthodontics is growing increasingly. Some reasons for this growth include easy insertion and removal of the miniscrew without irreversible changes, [1] immediate loading, [2] low cost of the instruments, and shorter duration of the treatment. [3][4] One of the requirements of immediate loading is primary stability [5][6][7] which is influenced by several factors including the design of the miniscrew, implant size, insertion angle, insertion torque, force angle, and the amount of applied force. [8] Design of the miniscrew is characterized by some factors such as the length and diameter of the miniscrew, thread shape, pitch, and depth. [9] Different thread shapes have been introduced, the basic forms of which are square, Vshape, buttress and reverse buttress. [8] Attempts to maximize the stability while minimizing the placement torque has led to the development of smaller miniscrews, which would broaden their clinical use.
Many studies on the design of orthodontic miniscrew and few on orthodontic miniscrew thread shapes have been conducted. [5,[8][9] Gracco et al. conducted an in vitro study to evaluate the effect of thread shapes on the pullout strength of the miniscrews. They designed four types of thread named as buttress, 75 joint profiles, rounded, and trapezoidal. They concluded that the thread design influenced the resistance to pullout and consequently the primary stability of orthodontic miniscrews. [7] Migliorati et al. used thread shape factor to determine the relationships between geometrical characteristics and mechanical properties of the temporary anchorage devices. Their results showed that maximum insertion torque and load values of the pull-out test were statistically related to the depth and shape of the thread of the screw. [11] Duaibis et al. showed that different thread shapes had no effect on the stresses around the cortical bone. [12] However, some studies, carried out on dental implants, have indicated that a key factor for the success or failure of dental implants would be the type and the amount of the bone stress. [8,13] Kong et al. designed a finite element study to determine the optimal thread shape for an experimental cylinder dental implant.
Twelve 3D models of dental implants with different thread shapes were investigated. They concluded that some of the thread shapes had better stress distribution. [14] Liu et al. showed that the direction of orthodontic forces had no significant effect on the cortical bone stress. However some other studies did not support this finding. [15][16] So there is a controversy over the impact of thread shapes on the bone stress and stability. Regarding this gap in literature, we decided to carry out a study to determine the effect of different thread shapes and force angles on stress distribution of bone and miniscrew. In our study, finite element analysis was used to evaluate the effect of different miniscrew thread shapes and load angles on stress distribution around miniscrew and supporting bone.   Mises stress is widely used by designers to make sure whether their design withstands the given loaded condition. [8,18] The calculated numerical data were shifted into color band diagram for better understanding

Bone stress distribution
Generally, the amount of stress for all force directions thread shape (6.62 MPa). (Table 2) Table 3 shows the maximum shear stress.

Miniscrew stress distribution
The peak von Mises stress in the miniscrew was at the smallest diameter of the shank, especially in the part inserted in the soft tissue. Generally, toward the tip of miniscrew, this value was decreased. For the part of the miniscrew located within the cortical bone, the greatest amount of stress was in the narrowest part of the shank (Figure 4).
Applying the 0° force angle, the peak von Mises stress of the miniscrew was ranged from 122.62MPa to 132.06 MPa, the lowest and greatest amount were for V-shape 1(V-1) thread shape and reverse buttress 1 (R-1) thread shape, respectively. However, when the angle of force increased to 45 o , the peak von Mises str-   with the lowest amount for V-shape 1 thread shape and the largest for reverse buttress 3 (R-3) thread shapes.

Deformation
The maximum deformation in all groups was at the head of the miniscrew, whereas to the tip of the miniscrew, this value was reduced. (Figure 5) With the 0° force of angle loading, the maximum value was for square 1(S-1) thread shape (0.0116 mm) and the minimum was for V-shape 2 (V-2) thread shape (0.0154 mm). At the 45° force angle, the result showed the range was between 0.0070 mm and 0.0109 mm. The largest value was for square 1(S-1) thread shape and the minimum was for V-shape 1(V-1) thread shape (Table 5).

Discussion
This study was conducted to evaluate the effects of various thread shapes and force directions on the stress distribution in different parts engaged in the process of loading.
The purpose of having threads on the miniscrew is to enhance the initial contact and surface area, which will optimize the stress distribution in the contact areas between the bone and the miniscrew. [8,19] Various forms of screw thread can have different impacts on the initial stability. Some of the thread forms introduced for dental implants are the square, V-shape, and buttress. The V-shape thread, also known as fixture, is primarily used for fixing metal parts together, not to transfer the load. On the other hand, the buttress thread is resistant to pullout force and the square threads provide an appropriate surface area for transmitting compressive and intrusive forces. [20] Applying the 200-cN force with 45° and 90° angles showed that the greatest amount of stress was in the cortical bone which was significantly higher than that of the cancellous bone. This result might be due to the different modulus of elasticity between these two types of bone.
The result was similar to the previous studies. [21][22] However in this study; the thickness of soft tissue was also considered while it was not investigated in the study of Singh et al. Since the maximum shear stress criterion is more conservative than the von Mises, the results obtained by von Mises are larger than the results obtained by Tresca; therefore, we only discussed the von Mises stress.   Therefore, the risk of failure is higher in this area. This can be justified with the second moment of inertia of a cylinder that shows the peak stress is inversely proportional to the third power of the diameter. These findings were different from the result achieved by Singh et al. [9] due to the fact that they did not consider a space for the soft tissue and reported the maximum stress to be located in the neck of miniscrew. Meanwhile, our result was similar to the results yielded by the study of Liu et al. [15] According to these results, it can be suggested that the part of the miniscrew which is inside the soft tissue should have a larger diameter relative to the portion that is located inside the cortical bone. Simultaneously, the thread width of the soft tissue area should be reduced compared to those in contact with the bone.
The thread shapes did not have a significant impact on the miniscrew deflection under the horizontal loading. This value decreased with the reduction of the angle of force because the amount of horizontal force was reduced as previously described.
Like other finite element studies, this study had some limitations in the simulation. [8-9, 12, 15, 22-23] The structures in the models were assumed to be linear, homogeneous, and isotropic; while, real bone is neither homogeneous nor isotropic, [12,25] but we used these assumptions for simplicity and to compensate the lack of information on the bone behavior. The geometry of the bone block was simplified to a rectangular block instead of a jaw section. The soft tissue was not simulated; although its thickness was deliberated. We assumed that the friction coefficient between the miniscrew and bone was 0.2; it might be different for cortical and cancellous bones. Since the thread shape might affect the insertion of miniscrew in the bone, further studies on this subject are recommended.

Conclusion
Considering the limitations of this study, two conclusions can be drawn: first, different thread shapes did not affect the pattern of distribution and the amount of von Mises stress; second, different force angles affected von Mises stress.